Heretofore the insulation for coils used in rotary electric apparatus and the like has been formed by placing an insulating layer of tapes or sheets, mainly consisting of synthetic resin or natural mica, along an inner surface of a slot adapted to receive the coil. The coil is placed into the slot and then coated or impregnanted with an impregnant consisting of a suspension of a natural laminated mica in a solution of organo-silicon compound thereby to fill the gaps formed between the coil and the insulating layer as well as the inner gaps of the coil per se. With such insulation, however, the organo-silicon compound is thermally adversely affected at temperatures above about 300.degree. C. with undesired loss of bonding between the filled insulating medium and the insulating layer, with consequent reduction of the mechanical and dielectric strength of the insulation.